5642
J. Phys. Chem. A 2006, 110, 5642-5649
Singlet and Triplet Excited-State Interactions and Photochemical Reactivity of Phenyleneethynylene Oligomers P. K. Sudeep,† P. V. James,‡ K. George Thomas,†,‡ and Prashant V. Kamat*,† Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Photosciences and Photonics, Regional Research Laboratory (CSIR), TriVandrum 695 019, India ReceiVed: January 18, 2006; In Final Form: March 6, 2006
The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances have made them useful as molecular linkers in the light energy harvesting assemblies and molecular electronics devices. These linker molecules themselves possess rich photochemistry as evident from the relatively large yields of the excited singlet (0.5-0.66) and triplet (0.4-0.5) states of two model oligomers, 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). In particular, the long-lived triplet excited state is capable of undergoing deactivation by self-quenching processes such as ground-state quenching and triplet-triplet (TT) annihilation. The T-T annihilation occurs with a nearly diffusion-controlled rate (∼2 × 109 M-1 s-1), and ground-state quenching occurs with a rate constant of ∼6 × 107 M-1 s-1. The electron transfer from the excited OPE-1 and OPE-2 to benzoquinone as characterized from the transient absorption spectroscopy illustrates the ability of these molecules to shuttle the electrons to acceptor moieties. In addition, pulse radiolysis experiments confirm the spectroscopic fingerprint of the cation radical (or “trapped hole”) with absorption bands in the 500-600 nm region.
Introduction Oligo(p-phenyleneethynylene)s have emerged as molecular building blocks for the design and fabrication of optoelectronic systems such as organic light emitting diodes,1-3 solar energy conversion systems,4-8 and chemical/biological sensors.9-15 The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances provide unique characteristics to design a new class of molecular systems.4,16-20 Of particular interest are molecular electronics devices. A few prototype systems were fabricated by incorporating molecular rigid rods in semiconductor/metal hybrid systems. These include (i) single-molecule diodes by immobilizing them between both electrodes by sulfur-gold bonds,21 (ii) nanocell electronic memories,22 (iii) voltage-triggered conductance switching,23-25 and (iv) photovoltaic systems by functionalization of redox/photoactive molecules on to semiconductor surfaces.4-8 Recently Galoppini and co-workers4-8 have investigated, in detail, the dynamics of the electron injection from excited RuII-polypyridine sensitizers to TiO2 semiconductor through phenyleneethynylene bridging units. These studies suggest that the charge injection occurs over a long distance of 2.4 nm. The electronic coupling between RuIIpolypyridine and the phenyleneethynylene bridging unit is quite strong and thus assists in the fast electron injection into semiconductor nanoparticles.4 Electrochemical studies have confirmed that phenyleneethynylene bridge structures promote strong coupling between gold electrode and ferrocene, thereby promoting rapid electron transport over long distances.16,26 Although the potential applications of this class of molecules in various devices have been demonstrated, a detailed under* Corresponding author phone: (574)631-5411; fax: (574)631-8068; e-mail:
[email protected]. † University of Notre Dame. ‡ Regional Research Laboratory (CSIR).
CHART 1: 1,4-Bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-Bis((4-phenylethynyl)phenylethynyl)-2,5bis(hexyloxy)benzene (OPE-2)
standing of the excited-state behavior is yet to be established. Most of the physical studies published to date deal with aggregation induced emission changes.10,13,27,28 The photophysical properties of oligomers based on the aryleneethynylene architecture, with a 2,2′-bipyridine-5,5′-diyl (bpy) metal binding unit, was reported by Schanze and co-workers.29 The excitedstate characterization indicated a composite behavior of two excited-state manifolds. Charge transfer through terthiophene end-capped poly(arylene ethynelene)s has been studied by characterizing the behavior of cations and anions in a pulse radiolysis experiment.17 Characterization of the geometric and electronic properties of alkoxy-substituted derivatives of phenyleneethynylenes, in comparison with an unsubstituted system, (1,4-bis(phenylethynyl)benzene), was reported in an earlier paper.30 These studies indicate that phenyleneethynylenes exist in planar and twisted conformations in their ground state due to the nearly free rotation of the arene rings along the molecular
10.1021/jp0603637 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/07/2006
Phenyleneethynylene Oligomers axis. On the other hand, these molecular rods attain a planar configuration in the excited state. Based on the picosecond time-resolved studies, Beeby et al. concluded that 1,4-bis(phenylethynyl)benzene does not exhibit significant cumulenic/quinoidal character in its S1 state.31 Interestingly, recent time-resolved IR measurements indicate that an unpaired electron when conjugated in the phenyleneethynylene core is substantially delocalized over the π-system of the chromophore resulting in the formation of a partial cumulene-like structure.32 Although there are several reports indicating that phenyleneethynylenes communicate charge as well as excitation energy over long distances, no major effort has been made to characterize the excited-state behavior and redox properties of this class of molecules. We report herein the behavior of excited singlet and triplet state as well as the redox properties of two model oligo(phenyleneethynylene)s possessing dialkoxy substitution, namely 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). Experimental Section Materials and Methods. Phenyleneethynylenes OPE-1 and OPE-2 were synthesized according to the published procedure.33 Care was taken to purify these compounds using the recycling HPLC method.30 It is difficult to separate the trace amounts of impurities present in these compounds using conventional column chromatographic or crystallization techniques. If not purified carefully these impurities interfere with the photophysical measurements. All solvents were spectrophotometric grade. 1-Pyrenecarboxaldehyde purchased from Aldrich was recrystallized before use. All experiments were performed at room temperature, and the solutions were deaerated by bubbling with nitrogen. Absorption spectra were recorded with a Shimadzu 3101 spectrophotometer. Emission spectra were recorded using an SLM 8000 photon counting spectrofluorimeter. Emission lifetimes were measured using Horiba Jobin Vyon single photon counting system, and the fluorescence decay measurements were further analyzed using the IBH software library. Pulse radiolysis experiments were performed with the 8-MeV Titan Beta model TBS-8/16-1S linear accelerator. Cyclic voltammetric experiments were carried out using a BAS 100B electrochemical analyzer. The measurements were carried out using a standard three-compartment cell consisting of a working electrode (glassy carbon), a reference electrode (saturated calomel electrode, SCE), and a counter electrode (Pt). All solutions were made in acetonitrile containing 0.05 M tetra-n-butylammonium perchlorate. Laser Flash Photolysis. Nanosecond laser flash photolysis experiments were performed with a 355 nm laser pulse (5 mJ, pulse width 6 ns) from a quanta Ray Nd:YAG laser system. Femtosecond transient absorption experiments were conducted using a Clark-MXR 2010 laser system and an optical detection system provided by Ultrafast Systems (Helios). The source for the pump and probe pulses is the fundamental of the Clark laser system (775 nm, 1 mJ/pulse, fwhm ) 150 fs, 1 kHz repetition rate). A second harmonic generator is introduced into the path of the laser beam to provide 387 nm (3.20 eV, 150 fs) laser pulses for the pump. The pump beam is attenuated to 5 µJ/ pulse with a spot size of 2 mm (diameter) at the sample where it is merged with the white light incident on the sample cell with an angle
